Coal liquefaction has been practiced since the 1920s and 1930s, starting in Germany, up until the present shortages of available petroleum products during the current energy shortages. The early coal liquefaction techniques utilized large amounts of hydrogen and extremely high pressure conditions in order to provide liquid fuel products from coal. Both of these criteria provide for an unfavorable economic production scheme. More recently, coal has been liquefied in various catalytic and thermal solvent refining techniques. Coals which are amenable to such processing include the nonanthracitic coals such as bituminous, sub-bituminous and lignitic coals, as well as other organic materials, such as peat.
When coal liquefaction is performed either with or without solvent, but in the presence of an added catalyst, it is termed a catalytic coal conversion technique. When coal is liquefied without the addition of external catalysts, but potentially in the presence of catalytic materials indigenous to the coal, such a processing is referred to as a thermal coal liquefaction. It is known to catalyze coal liquefaction by various catalysts such a molybdenum, cobalt, nickel, iron and combinations of these elements and their compounds.
Coal liquefaction can also be accomplished either with or without a solvent for the raw feed coal material. In those processes utilizing a solvent, generally termed solvent refining of coal, the solvent is typically recovered from the downstream product separation zone and is recycled for further use in the process. It is also known to recycle SRC specifically for those processes which utilize a solvent to effect the coal liquefaction. SRC is an abbreviation of the term solvent refined coal and is recognized in the art as constituting that portion of the nondistillable, pentane-insoluble products of the solvent refining of coal which portion is soluble in tetrahydrofuran, creosol or pyridine. When SRC is recycled in a coal liquefaction process, the second or subsequent passes of the SRC through the process are presumed to effect the further conversion of the SRC material to pentane-soluble and distillable products.
In recycling hydrogen donor solvent, the prior art has performed various separations and purifications of the recycle solvent in order to benefit the reaction to which the solvent is introduced. For example, U.S. Pat. No. 4,056,460 teachs the beneficial effect of recycling or adding amines and phenols as coal solvent constituents. A process is described in which phenols and amines are extracted from coal liquids and are recycled to the liquefaction zone. The patent identifies the phenolic functionalities in asphaltenes and states that although asphaltenes should not be recycled to the liquefaction reaction, after a reduction in molecular weight, the corresponding phenols are beneficial recycle solvents.
U.S. Pat. No. 4,057,484 disclosed that the acid-base structures in asphaltenes must be decomposed with either acid alone or caustic alone before a portion of the asphaltenes are recycled as pasting solvent for coal liquefaction.
U.S. Pat. No. 4,081,351 discloses a process utilizing coal extract which is denitrogenated before going to a catalytic cracker reaction vessel in order to protect the catalyst in the hydrocracker from nitrogenous components.
U.S. Pat. No. 4,125,452 discloses a coal liquefaction process in which a recycle solvent is separated from phenol-containing material and the phenol-free solvent is rehydrogenated before recycle to a coal liquefier.
In U.S. Pat. No. 4,133,646 it is stated that a coal liquefaction process, wherein 3-50% of phenols are recycled as solvent, is the desired goal of the patent process.
In addition, U.S. Pat. No. 4,472,263 to the same inventors herein and of common assignee discloses that recycle solvent in a solvent refining of coal can be treated to remove the phenol and nitrogenous base constituents therefrom. Further, this patent discloses that light SRC or predominantly benzene-soluble, solvent-refined coal may be recycled to the slurry mix tank of the coal liquefaction scheme of that patent. No examples are given to demonstrate the result of such an SRC recycle. The issue date of that patent is within one year of the filing date of this present invention.
In U.S. Pat. No. 4,411,766 it is disclosed that solvent-refined coal or SRC can be recycled as shown in FIG. 2 of that patent or as only the light solvent-refined coal as shown in FIG. 1 of that patent.
The present invention improves upon the teachings of the prior art wherein a novel combination of treated recycle solvent and SRC or a portion thereof is recycled in a catalytic process to provide an unexpected increased oil-make in the coal liquefaction or solvent refining process.